Current heavy-duty diesel engines put a moderate amount of soot (a form of unburned fuel) into the oil pan. This soot is generated due to the fuel hitting the cold cylinder walls and then being scraped down into the oil sump when the pistons reciprocate in the cylinders. Up until recently, the nitrous oxide emission regulations in the USA and other countries have been high enough that the fuel injection timing could be such that the level of soot generated was not high. In typical applications, the soot level would be under 1% (by weight) of the engine oil at oil drain time. At these low levels, soot in the oil does not cause any wear problems.
Recently, there has been a move to significantly lower nitrous oxide emissions which requires much retarded fuel injection timing, which significantly increases the amount of soot being generated. At reasonable oil drain intervals, the soot level may be as high as 4 or 5% with retarded injection timing. When the soot level gets this high, lubrication at critical wear points on the engine becomes so poor that high wear results, significantly decreasing the miles to overhaul and causing high operator expense.
Thus, the engine manufacturer has two choices, suffer very high warranty costs and low miles to overhaul, or significantly lower oil drain intervals to keep high soot levels out of the oil. Neither of these choices is desirable, so there is a current strong need to have a means of getting the soot out of the oil, the subject of this invention.
A problem with removing the soot from oil is that it is very small in size--around 0.1 to 2.0 micrometers. To remove such small particles from oil using barrier filtration is not feasible due to the large filter size required and the very high probability that the filter will become plugged very rapidly due to trying to filter to such a fine level.
One way that is feasible to remove the soot from the oil is by using a centrifuge, a device that removes the soot from the oil using centrifugal force. This type of device is used to separate blood constituents from blood and has many other applications in typical laboratory applications. The use of a centrifuge for an engine brings a requirement of doing it in a very inexpensive and reliable manner with the centrifuge being easily changed at oil change time. Heretofore, centrifugal filters have not been able to sufficiently remove soot from oil, sufficiently retain the soot, nor reliable enough for use in engine and vehicle applications.